A storage server is a computer that provides storage service relating to the organization of data on writable, storage media, such as non-volatile memories and disks. A storage server may be configured to operate according to a client/server model of information delivery to enable many clients (e.g., applications) to access the data served by the system. A storage server can employ a storage architecture that serves the data with both random and streaming access patterns at either a file level, as in network attached storage (NAS) environments, or at the block level, as in a storage area network (SAN). Storage servers (e.g., monolithic, non-distributed storage servers or clustered storage servers) store data on various types of non-volatile storage media such as, for example, relatively high latency (i.e., longer access times) hard disk drive devices (HDDs) and relatively low latency (i.e., shorter access times) solid-state devices (SSDs) such as flash memory or DRAM.
HDDs generally provide good streaming performance (e.g., reading of large sequential blocks or “track reads”) but do not perform well on random access (i.e., reading and writing of individual disk sectors) due to slow access times. SSDs on the other hand, are more suitable for random and frequent memory accesses because of the relatively low latency. SSD data storage devices use solid-state memory, such as non-volatile flash memory to store persistent data. With no moving parts, solid-state devices do not have mechanical delays resulting in the high latency experienced by HDDs and seek time is decreased significantly, making the SSDs very fast. SSD memory has this and other advantages over traditional disk drives. However, SSD memories are generally more expensive than HDD memories and have a shorter operational lifetime due to wear and other degradation.
Many storage servers use a combination of HDD and SSD memories. The SSD memories may be used for caching and allocated based on a requirement to meet service level objectives during peak usage. As a result, the expensive SSD memory may be over-allocated, and therefore underutilized, much of the time. As such, there are a number of challenges and inefficiencies found in traditional storage servers.